European Rosetta spacecraft poised for comet crash landing


Artist's impression of ESA's Rosetta spacecraft shortly before hitting Comet 67P/Churyumov–Gerasimenko on 30 September 2016. Illustration credits: ESA/ATG medialab.
Artist’s impression of ESA’s Rosetta spacecraft shortly before hitting Comet 67P/Churyumov–Gerasimenko on 30 September 2016. Illustration credits: ESA/ATG medialab.

The European Space Agency’s Rosetta spacecraft closed in Thursday for a deliberate crash landing on the surface of comet 67P/Churyumov-Gerasimenko on Friday, a slow-motion kamikaze plunge to bring the enormously successful mission to an end after more than two years of unprecedented close-range observations.

Flying in tandem with the comet some 356 million miles from the sun, Rosetta and its quarry are now moving beyond the range where the spacecraft’s solar panels can generate enough energy to power all of the probe’s instruments and subsystems.

“We’re going farther and farther away from the sun,” said Sylvain Lodiot, Rosetta operations manager for ESA. “Already now on the instrument side, we’re doing power sharing because there’s not enough power to maintain all the instruments active at the same time. In a few weeks, we won’t have enough power to operate the spacecraft. If we did nothing, at some point the spacecraft would just switch off.”

Instead, mission managers decided to move Rosetta in for a descent to the surface of 67P/Churyumov-Gerasimenko where it will forever rest within a half mile or so of the small Philae lander it dropped off in November 2014.

Designed to anchor itself in the comet’s icy crust, Philae famously bounced off, bounced again and ended up, two hours later, on its side in what its camera showed was a heavily shadowed, extremely rocky area near what appeared to be the side of a cliff.

In August, Rosetta’s powerful camera finally found Philae, beaming back a photograph of the wayward lander lodged as expected in rocky terrain.

Now, Rosetta will join its partner in science, collecting data from within a few hundred feet of the comet’s surface and beaming it back to Earth before impact around 12:20 p.m. BST Friday (plus or minus 20 minutes).

The final rocket firing to set up the terminal descent was expected about 14 hours before impact. At that point, the approach velocity was expected to be a sedate .67 miles per hour. By the time it reaches an altitude of about 1.2 miles above the surface, the comet’s weak gravity will have accelerated the spacecraft to about 1.3 miles per hour.

Finally, Rosetta will hit the surface at about 2 miles per hour, or walking pace. But that’s more than fast enough to doom a spacecraft that was never designed for landing.

“From an energy point of view, it will be a soft landing,” said Andrea Accomazzo, Rosetta flight operations director. “But Rosetta is not designed to land, so there will be some energy dissipation.”

Rosetta’s long solar panels will collapse or bend to the surface and instrument booms likely will crumple as the spacecraft hits and then rebounds in the comet’s modest gravity.

“For sure, Rosetta will sort of bounce and tumble on the surface of the comet, it will not stick immediately to the surface,” Accomazzo said. “But it will not bounce back into orbit.”

Software was loaded earlier to shut the spacecraft’s systems down at the moment of touchdown, switching all of its systems to standby. Even if its radios remained on, Rosetta would not be able to continue operation.

“Once we are landed, there’s absolutely no chance to be able to communicate with Earth any more,” Lodiot said. “Just to give you an example, if the high gain antenna is off pointing more than half a degree, there’s no communication possible anymore.”

And so, after a voyage spanning more than 12 years and nearly five billion miles, one of the most scientifically productive space missions ever launched will come to an end with Rosetta and the Philae lander forever moored to 67P/Churyumov-Gerasimenko as it continues its lonely trek around the sun.

Discovered in 1969, 67P/Churyumov-Gerasimenko circles the sun in an elliptical orbit extending nearly 500 million miles from the sun at its far point — beyond the orbit of Jupiter — to a point between the orbits of Earth and Mars some 115 million miles from the sun. The comet measures a few miles across and rotates every 12.7 hours.

Made up of material left over from the birth of the solar system 4.6 billion years ago, 67P/Churyumov-Gerasimenko is a frozen remnant of the material used to build the sun and its retinue of planets, a representative sample of the countless small bodies that may have seeded Earth with the organic compounds necessary for life.

Rosetta’s observations have revolutionized comet research. Contrary to some pre-arrival theories, Rosetta found that the water locked up in 67P’s icy crust is not the same type as found in Earth’s oceans, indicating comets may not be responsible for building up the planet’s oceans through countless impacts.

Analysis of surface elements and compounds shows 67P is, in fact, an ancient body, a relatively dry, highly porous “rubble pile” riddled with organic compounds, including the amino acid glycine, which can form in the absence of water.

“Comets do not contain life, that’s for sure,” said Kathrin Altwegg of the University of Bern. “But comets could have actually sparked life. Imagine you throw 67P into the ocean. Then it starts to melt, molecules become mobile, they can react with water to form other amino acids, they can react with each other and maybe that’s how life started on Earth.”

During 67P/Churyumov-Gerasimenko’s approach to the sun, Rosetta observed chunks larger than a yard across being blown away from active regions. It also has observed large pits as wide as a football field and equally deep, with walls made up of so-called “goosebumps” about 10 feet in diameter that may be the original building blocks of the comet.

During its descent Friday, Rosetta will look into one of these pits with its high-resolution camera to learn more about the intriguing structures.

“We are desperately waiting for the last scientific data coming down from Rosetta,” Accomazzo said. “It’s a unique opportunity to observe the comet from very, very close distances. Instruments will be able to sniff the environment around the comet from a few hundred meters.

“OSIRIS, the camera, will have a resolution which is fantastic. We should be able to look into this pit, to look at the internal structure of the comet. But the final minutes we’ll simply be looking at the radio signal picked up by our spectrum analyzers. Once this disappears, we’ll know it’s the end of the mission.”

Launched nearly 13 years ago, on 2 March 2004, Rosetta carried out four flybys of Earth and Mars to pump up its velocity enough to move out into deep space on a trajectory to reach 67P/Churyumov-Gerasimenko.

The mission plan called for catching up with the comet near the outer reaches of its orbit so Rosetta could follow it into the inner solar system and back out again.

By the summer of 2011, after flying past two asteroids, Rosetta was too far from the sun for its solar arrays to generate enough electricity to power all of its instruments and subsystems. At that point, flight controllers ordered the spacecraft to put itself into electronic hibernation, shutting down all non-essential systems, including its radio.

For the next two-and-a-half years, Rosetta flew in silence, constantly closing the gap with 67P/Churyumov-Gerasimenko. Finally, on 20 January 2014, Rosetta woke itself up and phoned home, ready for the final leg of its long space odyssey.

Rosetta finally matched orbits with its quarry on 6 August 2014. Philae was dropped to the surface three months later, on 12 November.


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